1. Field of the Invention
The present invention relates to a discharge lamp lighting circuit, and more particularly to a discharge lamp lighting circuit incorporating noise countermeasures.
2. Description of the Related Art
There is known a structure in which a DCxe2x80x94DC converting circuit, a DC-AC converting circuit and a starting circuit are provided in the lighting circuit of a discharge lamp, such as a metal halide lamp.
In order to suppress the influence of a noise generated by the DCxe2x80x94DC converting circuit, there is a method of causing a switching frequency to have a fluctuation. By changing the noise into a white noise that can be dispersed, it is possible to reduce an influence on an apparatus for receiving an electric wave, such as a radio wave.
For example, in the case in which PWM (pulse width modulation) control is to be carried out in the DCxe2x80x94DC converting circuit, a fluctuation having a certain width is given to a switching frequency and is defined to be a predetermined frequency so that the generation of a beat noise can be prevented. In other words, in the case in which the switching frequency has a fixed value, a noise is generated at a harmonic frequency.
FIGS. 14 and 15(a) and 15(b) are diagrams for explaining the application of a fluctuation related to a switching frequency, FIG. 14 being a diagram showing the main part of a control circuit (a sawtooth wave generating section) and FIGS. 15(a) and 15(b) being schematic waveform diagrams.
In this case, the PWM control is supposed for the switching control of the DCxe2x80x94DC converting circuit, and a resistor b and a capacitor c are connected to a terminal xe2x80x9cRT/CTxe2x80x9d of a sawtooth wave generating circuit as shown in FIG. 14. The terminal xe2x80x9cRT/CTxe2x80x9d is provided for determining the frequency of a sawtooth wave and is connected to a node of the resistor b and the capacitor c. More specifically, one of the ends of a constant voltage source representing a reference voltage xe2x80x9cVrefxe2x80x9d is connected to one of the ends of the resistor b and the other end of the resistor b is connected to the terminal xe2x80x9cRT/CTxe2x80x9d and is grounded through the capacitor c.
In order to change the switching frequency, it is preferable that the resistance value of the resistor b to be connected to the terminal xe2x80x9cRT/CTxe2x80x9d should be changed (the resistor b is shown as a variable resistor in the drawing) or a current (a source current) to flow to the capacitor c should be changed. Consequently, the inclination of the sawtooth wave can be varied.
In FIGS. 15(a) and 15(b), xe2x80x9cSAWxe2x80x9d indicates a sawtooth wave and xe2x80x9cCVxe2x80x9d indicates a control voltage, and the switching control of the DCxe2x80x94DC converting circuit is carried out based on a signal xe2x80x9cScxe2x80x9d (which has an H level if CV is larger than SAW) determined by a level comparison therebetween.
The frequency of Sc is low because the inclination of the sawtooth wave SAW is small as shown in FIG. 15(a), and the frequency of Sc is increased because the inclination of the sawtooth wave SAW is very great as shown in FIG. 15(b).
The switching frequency is determined by the frequency of Sc. For example, therefore, it is preferable that a frequency should be controlled to swing between an xe2x80x9cXxe2x80x9d kHz (kilohertz) and xe2x80x9cYxe2x80x9d kHz and xe2x80x9cZxe2x80x9d Hz should be set to be the frequency of a fluctuation by the application of the fluctuation to the frequency (the frequency of a change related to a current supplied to the capacitor c is defined to be xe2x80x9cZxe2x80x9d Hz and a degree of the change is defined to be (Yxe2x88x92X)/X).
As indicated by FIGS. 15(a) and 15(b), referring to the fluctuation in the PWM control, the switching frequency is changed with a variation in the inclination of the sawtooth wave SAW. In the case in which the level of CV is constant, the duty cycle (or duty ratio) of Sc is almost constant. As the fall of the sawtooth wave SAW is very great, the stability of the duty cycle is increased.
However, the suppression of a beat noise in the related art circuit structure is a matter formed at the sacrifice of the efficiency of the circuit. Accordingly, there is a problem in that a power loss is generated. In other words, an increase in the efficiency and the suppression of the beat noise are contrary to each other.
Therefore, it is an object of the invention to cause an increase in an efficiency and the suppression of a beat noise to be compatible with each other in a discharge lamp lighting circuit.
The invention provides a discharge lamp lighting circuit comprising a DCxe2x80x94DC converting circuit for converting a voltage input from a DC power source into a desirable DC voltage, a DC-AC converting circuit provided in a latter stage of the circuit, and a control circuit for controlling to turn ON a discharge lamp, in which the following structures are employed.
(A) The DCxe2x80x94DC converting circuit has a switching element to be connected to a transformer and a primary winding thereof, and control is carried out in such a manner that the transformer stores energy while the switching element is set in an ON state in response to a signal sent from the control circuit, the energy is output from a secondary winding of the transformer while the switching element is set in an OFF state in response to a signal sent from the control circuit, and the switching element is turned ON when the energy is completely output from the secondary winding in a stable lighting state of the discharge lamp.
(B) An output current or power related to the discharge lamp is controlled for an ON period of the switching element in response to a signal sent from the control circuit.
(C) Fluctuation generating means for applying a fluctuation to the control of the output current or power in the (B) is provided.
According to the invention, therefore, a fluctuation is applied in the control of the output current or power related to the discharge lamp. Consequently, the switching frequency can be changed to suppress a beat noise. In addition, control is carried out to always turn ON the switching element when the energy stored in the transformer constituting the DC-AC converting circuit is completely output from the secondary winding. Consequently, it is possible to reduce a power loss caused by turning ON the switching element. Thus, it is possible to prevent the efficiency of the circuit from being deteriorated.